In credit cards or ID cards, authentication information is recorded. A decision as to whether the information is authentic or not has been made by use of a magnetic recording region arranged in the rear face of the cards, a hologram attached to the front face thereof, or some other member. For example, authentication by use of a hologram image is disclosed in U.S. patents listed as Patent Documents 1 and 2 below.
In a passport disclosed in Patent Document 3 listed below, a latent image which cannot be recognized with naked eyes without using any polarizing plate is formed in a layer made of a polymeric liquid crystal material, and a reflecting layer is formed beneath the layer. The document discloses a method of radiating polarized light thereto and observing the reflected light through the polarizing plate, thereby authenticating a pattern formed as the latent image.
A means for forming a latent image in a retardation film may be a method of giving a part of the retardation film a heat for the glass transition point or higher, thereby lowering the phase difference (the degree of the alignment of the molecules) of the part, or a method of applying a chemical liquid wherein the retardation film can be dissolved or swelled, thereby lowering the phase difference of the region, as disclosed in Patent Document 4 listed below.
An optical element disclosed in Patent Document 5 listed below is in accordance with a method of changing the azimuthal angle of the optical axis of its retardation layer to form a latent image, and observing the image through a polarizing plate, thereby attaining an authentication.
Patent Document 1: U.S. Pat. No. 5,574,790
Patent Document 2: U.S. Pat. No. 5,393,099
Patent Document 3: Japanese Patent Application Laid-Open (JP-A) No. 2001-232978
Patent Document 4: JP-A No. 8-334618
Patent Document 5: Japanese Patent Application National Publication (JP-A-) No. 2001-525080